Description of Prior Art
It is considered to be important to effectively convert the high frequency electric power to mechanical vibration. For this purpose, there have been proposed various wire bonder systems in which ultrasonic vibration is impressed to a very fine wire through a tool attached to the wire bonder system and when said fine wire is accurately ultrasonic bonded onto the surface to be bonded, high frequency electric power is supplied from electric power unit to a transducer with stability.
FIG. 5 indicates an example of ultrasonic bonding of a general wire bonder system. The wire bonder system comprises an electric power unit 1, a transducer 2 to enlarge an effective component of mechanical vibration generated in the electric power unit 1 and a tool 4 for ultrasonic bonding of the conductive wire 3 to a bonding surface using said enlarged effective component from the enlarged side of the transducer. The wire bonder system is housed in a wire bonding machine (not shown) and is applied for wiring semiconductor parts 6 on a table 5.
Said electric power unit 1 comprises an oscillator 1a to generate high frequency signal and an amplifier 1b to stabilize the electric power of the signal to a predetermined voltage value or electric current value, tuning the frequency of the signal automatically to a value adaptable to the vibration property of the transducer 2. High frequency electric power is always distributed to the transducer 2 with optimum frequency, voltage value or electric current value through the wiring.
The transducer 2 is provided with a piezoelectric transducer 8 to convert the supplied high frequency electric power to mechanical vibration at its one end. The transducer 2 is also provided at its opposite end with a horn 10 to enlarge amplitude of the extracted effective component of vibration through a cone 9 to extract an effective component of converted vibration. The transducer 2 is supported at a fulcrum 9b of the wire bonding machine not indicating total weight of itself and the enlarged side of the transducer is moved forward and backward (arrow 12) by rotating said enlarged side upward and downward and leftward and rightward (arrow 11) by predetermined distance.
The horn 10 is provided with a clamp portion 10a to fit the tool 4 at the enlarged side of amplitude. The tool 4 is inserted with its installing side into the clamp portion 10a and fixedly tightened to it by means of screw 13.
The tool 4 uses either a capillary 4 which is a cylindrical form having a fine tubular hole 4a in the axial direction or a wedge which is a semicylindrical form having a fine tubular hole slanted by predetermined angle to the bonding side in the axial direction. The tool 4 is constructed with its bonding side sharpened and to fix the opposite side to the clamp portion 10a of the horn.
The piezoelectric transducer 8 applies either an electrostricted piezoelectric transducer which is composed of piling up a plural number of thin plates using a piezoelectric effect member and a plural number of electrodes alternatively, or a magnetostricted piezoelectric transducer which is composed of winding up coils around a plural number of cores using a magnetostricted effect member. The electric power unit 1 supplies an electric current stabilized to the predetermined value to electrostricted piezoelectric transducer and supplies an electric current stabilized to the predetermined value to the magnetostricted piezoelectric transducer.
The table 5 houses a heater to assist heating the semiconductor parts 6 placed on the table.
The semiconductor part 6 is composed of a substrate 6 of printed wire with multi-conduction pattern and an integral circuit chip 6c provided with a plural number of electrode 6b made of aluminum, gold vapor deposition and hybrid circuit die bonded on the surface of the substrate 6. Very fine conductive wire 3 is used to wire between said conduction pattern and aluminum electrode 6b.
Next, general treatment of wire bonder system and behavior thereof are explained. The wire 3 used for wiring is made of very fine gold wire, aluminum wire and in rare case copper wire. When gold wire or copper wire is used the capillary is attached to the transducer as the tool and when other wiring materials are applied wedge is attached to the transducer as the tool. When gold is used for wire 3 the semiconductor part 6 is heated beforehand by the heater (not shown). The following is an explanation of the case that gold is used as capillary.
The wire bonding machine (not shown in figure) is switched on. High frequency electric power is generated in the electric power unit 1. The semiconductor part 6 on the table is heated to the predetermined temperature. On the other hand, the wire is pulled out from a wire supporting equipment (not shown) and it is inserted into the tubular hole 4a of the capillary 4 from the fixed side and protruded from the bonding side. The tip of the wire 3 is made round to be adaptable to ball bonding by means of melting torch. After the preparation step of the foregoing is over practical wiring operation is started.
The bonding side of the capillary 4 is moved to the aluminum electrode 6b provided on the integral circuit chip 6c by controlling the position of wire bonding machine. The gold wire 3 protruded from the bonding side of the capillary is contacted to the surface of the electrode 6b. The gold wire 3 is then pressed to the electrode 6b by adding a predetermined pressure (arrow) 14 in the axial direction of the capillary 4.
Next, generated electric power of high frequency is supplied from the electric power unit 1 to the transducer 2. The supplied electric power is converted to a mechanical vibration of the same high frequency in the axial direction of the transducer 2. The effective component of said converted vibration is amplified and transmitted to the capillary 4.
All components comprising the transducer 2 have proper resonance frequency adaptable to known physical properties, respectively. The axial length of piezoelectric transducer 8, cone 9 and horn 10 are set up to the predetermined length and these parts resonate to the predetermined freqency.
FIG. 6 shows transmission of mechanical vibration of the general wire bonder system. The same reference numerals are used for the same parts disclosed in FIG. 5. Reference numeral 15 indicates a curve of amplitude when the transducer 2 including each component is working in normal condition. Reference numeral 16 indicates the axis of abscissas and the value of amplitude on this axis means theoretical zero.
Reference numeral 17 indicates the amplitude of the connecting portion of the piezoelectric transducer 8 and cone 9. Reference numeral 18 indicates the amplitude of the connecting portion of the cone 9 and horn 10. Reference numeral 19 indicates the amplitude of the connecting portion of horn 10 and capillary 4. Amplitude of resonance at each point being maximum, mechanical vibration is transmitted most effectively. On the other hand, reference numeral 20 indicates amplitude at the piled surface of the transducer member to be used for piezoelectric transducer. Reference numeral 21 indicates amplitude at the supporting point 9b of the transducer 2. At these points amplitude adversely becomes zero, namely, displacement of resonance is zero, thus converted mechanical vibration is prevented from being adversely converted to electrical vibration or being leaked outside from the supporting point 9b.
Accordingly, the transducer 2 provided with the capillary 4 most effectively convert the electric power supplied thereto from the electric power unit 1 and can transmit the vibration to the capillary 4 with the least loss. In other words, impedance of electric power from the electric power unit 1 becomes minimum.
There have been disclosed problems as described hereinunder when the capillary 4 is fitted to an enlarged portion of the general wire bonder system and the semiconductor part 6 is wired with the conductive wire 3.
FIG. 7 shows the kind of capillaries. Total length and diameter are defined as international standard as follows:
S type: total length 0.250 in/6.35 mm PA1 L type: total length 0.375 in/9.525 mm PA1 XL type: total length 0.437 in/11.1 mm PA1 XXL type: total length 0.470 in/12.0 mm PA1 16 mm type: total length 0.630 in/16.0 mm PA1 (1) Electric power is not effectively transferred to vibration energy. PA1 (2) Frequency becomes unstable.
Among them, diameter of 1/16 in. is principal. Very rarely there is 1/8 in.
Various kinds of configuration of the tip of the capillary are used.
FIG. 7(a) indicates a tip of taper of 30.degree.. FIG. 7(b) indicates a tip of taper of 20.degree.. FIG. 7(c) indicates a tip having a diameter twice as much as (a) . FIG. 7(d) indicates a tip of taper of 30.degree. with a bottle neck thereon.
FIG. 8 shows cross-sections of capillary and wedge. FIG. 8(a) is a cross-section taken from I--I line of FIG. 7(d), namely, bottle neck type. The tubular hole 4a of the capillary 4 has an inside cone tapering from body to tip of bonding side, said inside cone having an angle of 10.degree. as principal, and rarely 15.degree., 20.degree. and 24.degree..
FIG. 9 is a drawing to explain interference between the bonding side of the capillary and the package wall of the semiconducter part. In practical case, the semiconducter part has a package 6d to compose outside contour. A lead 6c passing through package 6d from an outside contour is provided. The wire 3 to be wired on the surface of the lead 6c is bonded by means of ultrasonic bonding. The capillaries of FIG. 7 are placed against the wall of package 6d facing it with bonding side. It is, therefore, not possible to reach the most optimum bonding position. The triangle portion including arms C1 and C2 which interfere the package 6d should be removed.
FIG. 10 is partially expanded drawing to explain the status at the bonding side of the capillary.
FIG. 10(a) partially eliminates the bonding side of the capillary from one side. It does not interfere with the part of triangle portion of the package 6d of FIG. 9. FIG. 10(b)-(d) present a bottle neck state eliminating the bonding side of capillary from a plural number of direction or all direction. These capillaries do not interfere with various packages but also not interfere with the wire already bonded to high density print wiring and electrodes. Namely, it is possible to reach the optimum bonding position.
The raw material of the capillary was traditionally made of glass when the wire bonding technique for semiconductors was developed but recently the glass has been replaced with tungusten carbide and titan carbide which were taken to process desired properties. These carbide materials are high in strength but their useful life as a capillary is short in comparison with glass. For this reason, nowadays ceramic material is particularly used.
More improved ceramic materials of high density and longer useful life have been developed. From high density ceramic material to multicrystal ruby and sapphire, and then from monocrystal ruby and sapphire to silicon-aluminum-nitride and hybrid ceramic material with a sapphire tip have been developed. These improved materials are used in accordance with requirements of cost and useful life.
In place of the capillary, a wedge of semi-cylindrical tube indicated by FIG. 7(e) is used. FIG. 8(b) is a cross section partially enlarged from II--II line of wedge in FIG. 7(e). The tool 4 using this wedge is provided with a tubular hole 4a inclined from its axial direction. The very fine wire 3 made of aluminum is inserted through the hole 4a and protruded to bonding side. The wire 3 is cut to the predetermined length by the tool. It is possible to continuously wire to a multi-number of electrodes and leads on the semiconductor parts in a particular direction.
There are standard total length and diameter in the international market of the wedge. Total lengths of 3/4 in. and 1 in. are common and diameters of 1/16 in., 1/8 in. and 3/32 in. are most widely used. Rarely, diameters of 2 mm and 3 mm are sold. Tungsten carbide is applied as a raw material of these wedges.
On the other hand, tables to place semiconductor to be wired are selected in accordance with kinds of semiconductor or wiring work. A table of rugged surface is sometime used. For these tables of rugged surface a capillary or wedge is protruded from the transducer to avoid such rugged portions.
As described in the foregoing, a capillary or wedge of various dimensions and kinds is applied in accordance with requirements of semiconductor parts and wiring, respectively.
Accordingly, the transducer is required to effectively transmit mechanical vibration by means of respective tools. As described, the value zn of impedance of electric power supplied from the electric power unit is required to be maintained low and stable.
Next, how to fit these tools to the transducer is explained.
FIG. 14 is a flow sheet to explain how to fit these tools to transducers in the prior art. Preparation step 31 to set the condition under which tools are mounted to the transducer is provided. Following the step 31, next step 32 determines whether the step should be avoided or not.
In the preparation step 31, it is required to set up ln length protruded from the transducer to a beginning value L adaptable to the condition of wiring work. Torque value tn of the screw to tighten the tool onto the transducer is set up to a predetermined value T which is adaptable to the tool.
In preparation of avoidance step 32, it is recommended to provide an adjustment step 33 to fix the tool onto transducer and following to this adjustment step 33 a trial step 34 to tentatively function the transducer. Following this trial step 34, a determination step 35 to judge the working result is provided. A compensation step 36 deriving from the determination step 35 and coming back to the adjustment step 33 to change the mounting condition of the tool in accordance with the result obtained in the determination step 35 is provided.
In the adjustment step 33 the tool is protruded by predetermined length of ln beyond the transducer then the screw is fixed by the predetermined torque tn and the tool is tentatively fixed to the transducer.
In the trial step 34 the wire is contacted with its spherical tip to the surface of trial electrode (not shown) in the semiconductor parts. Further, a predetermined pressure is added to the tool in the axial direction and the spherical tip of the wire is contacted to the bonding side of the tool. Electric power of high frequency from the electric power unit is supplied to the transducer and the wire is tentatively bonded. Said predetermined pressure is removed at the determination step.
At the determination step 35, whether the quality of wire bonded at the trial step 34 has reached the standard or not is inspected by skilled operator. Inspection is repeated from the adjustment step 33, the trial step 34 and to the determination step 35 through the compensation step 36 until reaching standard.
When the result obtained in the determination step 35 is determined to be repeated, the compensation step 36 compensates the protruded length ln to ln+1 and said compensated length is applied in the adjustment step 33.
Reference numeral 37 indicates beginning of operation and reference numeral 38 indicates end of operation.
However, in a practical wiring process of semiconductor parts the impedance of supplied electric power sometimes become very unstable and it causes inaccurate bonding result due to the change of the applied tool to different ones. It also happens that the impedance becomes large and wiring work discontinues.
FIG. 11 shows the relation between the value of impedance and the location to mount the capillary. Vertical axis indicates the value zn of the impedance of supplied electric power from the electric power unit in the unit of ohm and horizontal axis indicates the protruded length ln at the bonding side of capillary inserted into the tip of the transducer in the unit of m/m. Reference numeral 23 is a curve indicating the value of zn of impedance of electric power when the protruded length of the capillary is changed. When the length of said protruded length ln is set to approximately 4.5 mm, the slant line portion 27 indicates that the maximum impedance value of 24 is generated.
Reference numeral 25 indicates a curve of polycrystal ruby and when the length of said protruded length is set to approximately 6 mm, the slant line portion 28 indicates the maximum value of 26.
In short, in a consecutive processing of wiring, a capillary made of high density ceramic material is protruded by 6 mm from the transducer. After bonding work has been done in stable condition the capillary is changed to polycrystal ruby with the same length to continue same bonding work and the value zn of impedance of supplied electric power becomes approximately maximum value of 26. It becomes impossible to supply with a fluent ultrasonic energy for bonding and wiring work discontinues.
When the length of the protruded capillary is determined so that the value of impedance zn may occupy maximum values of 24 and 26 the frequency of electric power supplied from the electric power unit cannot maintain the resonance frequency optimum to the characteristics of transducer and it is sometimes transferred to another frequency.
FIG. 12 and FIG. 13 are the drawings which explain the relation between the frequency of the electric power of high frequency supplied to the transducer from the electric power unit and admittance. The horizontal axis indicates that frequency f increases in the right direction and vertical axis indicates that admittance value y increases in the upward direction.
The admittance value y is converse of the impedance value zn of supplied electric power.
As mentioned in the foregoing, the electric power unit automatically adjusts the frequency f of supplied electric power so as to make the impedance value zn to be minimum, said frequency f being tuned to the resonance frequency F.sub.0 adaptable to the physical property of wire bonder system, thus said wire bonder system is used for stable bonding.
When the tool is mounted protrudently from the transducer with the length ln so that the impedance value zn of the supplied electric power becomes approximately maximum, the frequency F2 for which the impedance value zn becomes maximum (admittance value becomes minimum) in the vicinity of slant line portion 29 including the resonance frequency F.sub.0 of wire bonder system is generated. Thus, the resonance frequency F.sub.0 relatively disappears. With this regard, frequency F.sub.1, F.sub.3 to minimize the impedance value zn (admittance value becomes maximum) which is similar to resonance state at both sides are relatively generated in the slant line portion 30 in the vicinity of frequency F2.
On the other hand, the amplifier 1b of the electric power unit 1 is automatically adjusted to the frequency of the supplied electric power to mimimize the impedance value zn (namely, the admittance value y becomes maximum) in the vicinity of resonance frequency F.sub.0 as expected. Thus, in place of the resonance frequency F.sub.0 already disappeared in FIG. 13 the amplifier 1b is attracted and tuned to either of the frequency F.sub.1 or F.sub.3 which give a minimum impedance value zn (in other words, admittance value y is maximum).
In short, electric power of frequency which is far separated from the value expected to be adaptable to the vibration property of transducer is supplied. The energy of this electric power is converted to mechanical vibration through the piezoelectric transducer but it is not accurately transmitted in the transducer due to the reason described in the foregoing. This will cause a loss at the supporting point to support transducer, as well as a piled surface of vibration prevention.
As above mentioned, when the tool is changed in accordance with the use of wiring work, the supplied electric power is changed as follows:
The wire is inaccurately bonded.
It is, therefore, a subject of the present invention to present a wire bonding system to supply a high frequency electric power to transducer constantly to effectively convert said electric power to ultrasonic bonding energy.